1. Field of the Invention
The present invention relates to a rotation detecting apparatus for use in a length measuring apparatus for measuring the length of an object or length between two points, or a distance measure for measuring the distance between two points on a map.
2. Description of the Related Art
An apparatus is known which rotates a rotational disc along a line connecting two points and detects the amount of rotation of the disc to thereby measure the distance or length between the two points. Such a measuring apparatus is disclosed in, for example, Japanese Patent Disclosure Nos. 61-16970 and 61-149801.
U.S. Pat. No. 3,999,298 discloses such a measuring apparatus assembled in a wrist watch.
This sort of measuring apparatus is designed in a such a manner that part of the rotational disc disposed in a casing is exposed and, when the exposed part is pressed against and rolled on a map, for example, the disc's rotation is transmitted through a plurality of transmission gears to a rotational member provided within the casing. Therefore, the length or distance between any two points on the map can be measured by detecting the amount of rotation of the rotational member.
According to such a measuring apparatus, since the rotational disc always engages the transmission gears, the apparatus is likely to easily function even when unintended, thus resulting in undesirable malfunction. In addition, since the rotational disc is always partially exposed from the casing, the disc itself may easily be damaged.